1. Field of the Invention
Embodiments of the invention are most generally related to the field of signal pulse measurement. More particularly, embodiments of the invention are directed to optical and electrical pulse measurement apparatus and methods.
2. Background Discussion
Nanosecond-length pulses are used in a variety of applications including, but not limited to, e.g., LIDAR, remote sensing, and laser-based inertial confinement fusion (ICF). It is particularly advantageous to be able to accurately measure the pulse shape of high contrast laser pulses used in ICF. Shape contrast refers to the complex temporal shape of an optical pulse. For example, a pulse might rise rapidly to a low, relatively-flat, plateau-region followed by a slow ramp to the peak of the pulse (a Gaussian would not be considered a complex shape). The shape contrast is the ratio of the peak of the pulse to the minimum feature amplitude that must be controlled. Thus the contrast may be considered as the ratio of the peak to the plateau. Alternatively, if the pulse has a sharp spike on the leading edge, followed by a valley and then a ramp up to the peak, the contrast would be considered the peak to valley ratio.
It is known that streak cameras or photodiodes in conjunction with oscilloscopes, for example, can be used for the measurement of nanosecond-length pulses, however, these apparatus and associated measurement techniques have recognized shortcomings. For example, the relatively slow update rate of single-shot, high-dynamic-range streak cameras limits their usefulness in applications that require real-time monitoring. Exemplary ICF applications include real-time pulse shape adjustment and diagnosis of intermittent problems.
The usefulness of oscilloscopes for the measurement of nanosecond-length pulses with picosecond-scale features has been limited by device performance constraints related to, for example, insufficient vertical resolution and/or effective number of bits. These constraints further limit the dynamic range capability for sufficiently measuring high-contrast pulse shapes with the oscilloscope measuring apparatus.
A conventional method for reducing noise on periodic signals involves averaging temporally sequential events. The technique has the advantage of reducing the signal-to-noise ratio (SNR) by a factor of N1/2, where N represents the number of sequential events. However, the averaging process tends to wash out the non-repetitive, single-shot events, which becomes particularly important in the attempted diagnosis of intermittent failures. Moreover, acquisition speed in sequential averaging is reduced by a factor of N.
One reported approach to capture single-shot events involves actively replicating the pulse. The gain in an active fiber loop was used to maintain signal amplitude throughout the pulse train, but at the expense of amplifier noise added to the signal at every pass. Furthermore, the amplitudes of the resultant pulse train followed an exponential decay curve, limiting operation at high repetition rates.
Another reported approach to measure single-shot, ultra-fast optical waveforms involves compressing a large bandwidth laser signal and employing a fast optical gate to sample time-varying slices of different temporal portions of a replicated pulse train. This technique is inefficient in that it throws away a significant portion of signal photons in the measurement process.
The inventors have recognized the advantages and benefits associated with improved and new apparatus and methods for pulse measurement that address the shortcomings of currently available measurement apparatus and techniques as discussed herein and as further recognized by those skilled in the art. Moreover, the inventors recognize the advantages of being able to measure both optical and electrical pulses of nano-second duration and, further, the ability to measure pulse signal contrast having a dynamic range that exceeds the rated dynamic range of the particular measuring device.